To provide a plant for producing a solid fuel, wherein an amount of final disposal is reduced while reducing generation of harmful substances, and waste and sludge are multiply treated at a low cost.

A solid fuel-forming plant 1 from sludge includes a mixed waste treatment line 2 extracting combustibles including waste plastics from waste, a sludge treatment line 3 fermenting and drying the sludge to form fermented and dried sludge, a woody waste treatment line 4 forming a woody waste and woody chip from woody waste, a solid fuel producing line 5 producing RPF using the combustibles from the mixed waste treatment line 2 and the fermented and dried sludge from the sludge treatment line 3 and a boiler 6 supplying steam to the sludge treatment line 3. In the mixed waste treatment line 2, waste plastics of polyvinyl chloride is removed by an optical sorting device 35 after lightweight products sorted from the waste are washed and dehydrated by a washing and dehydrating device 30.